Articles Tagged ‘Link State Advertisements’

OSPF Adjacency & Neighbor Forming Process. OSPF Hello Messages, OSPF Database Updates via Link State Requests (LSR & LSU)

ospf-adjacency-neighbor-forming-process-hello-packets-lsr-lsu-1aThis is the second article of our OSPF series which describes how OSPF routers perform neighbor relationship and adjacency. We’ll examine how OSPF discovers neighbors by sending Hello packets through the router interfaces and how it shares Link State Advertisements (LSAs) to form adjacencies and build its topology table. We’ll also examine the contents of OSPF Hello packets (Router ID, Hello/Dead Intervals, Subnet Mask, Router Priority, Area ID, DB & BDR IP Address, Authentication information) and more.

Our first OSPF article covered basic OSPF concepts - OSPF Topology & Routing table, OSPF Areas & Router roles, plus more. It is recommended users read the first article before continuing, to help refresh their OSPF theory.

How OSPF Forms Neighbor Relations

Once OSPF is enabled on a router interface, a Link State Database (LSD) is established and all interfaces running OSPF are added to this table to be used in Link State Advertisements (LSAs), OSPF then the begins neighbor discovery and forming adjacency process.

We’ll now take a closer look at both, neighbor discover and adjacency forming process:

R1 sends an initial OSPF Hello packet. R2 responds with an OSPF Reply Hello packet.

Figure 1. R1 sends an initial OSPF Hello packet. R2 responds with an OSPF Reply Hello packet.

Sending & Receiving OSPF Hello Messages

An OSPF router generates a Hello packet every poll interval -10 seconds for Peer-to-Peer (P2P) networks and 30 seconds for Non-Broadcast-Multiple-Access (NBMA) networks by default- and advertises it through multicast address 224.0.0.5 to all routers connected to its interfaces while it searches for potential OSPF neighbors. The Hello message contains a list of information needed to form an OSPF neighbor relation between two neighboring routers, the following a list of information contained the Hello messages:

  • OSPF Router ID. The router’s ID which is configured or automatically selected by OSPF (analyzed below)
  • Hello Interval Timer. Frequency upon which Hello packets are sent.
  • Dead Interval Timer. Defines how long we should wait for hello packets before we declare the neighbor dead.
  • Subnet Mask
  • Router Priority. Used to help determine the Designated Router (DR). Higher priority takes precedence. A configured Priority of 0 means the router will not become a DR or BDR.
  • List of reachable OSPF neighbors in the network.
  • Area ID
  • DR & BDR’s IP addresses (if exists)
  • Authentication Password (if configured)

Once a neighbor router (R2) running OSPF receives the Hello message, it runs a check on the above list.

The following conditions must be met for two routers to become neighbors:

OSPF LSA Types - Purpose and Function of Every OSPF LSA

Our previous article explained the purpose of Link State Update (LSU) packets and examined the Link State Advertisement (LSA) information contained within LSU packets. We also saw the most common LSA packets found in OSPF networks. In this article we’ll be diving deeper to analyse all eleven OSPF LSA Types using network network diagrams and examples to help understand when each LSA type is used and how they keep the OSPF network updated.

LSA Types - Quick Overview

Before we begin, let’s take a quick look at the different type of OSPF LSA packets we’ll cover:

  • LSA Type 1: OSPF Router LSA
  • LSA Type 2: OSPF Network LSA
  • LSA Type 3: OSPF Summary LSA
  • LSA Type 4: OSPF ASBR Summary LSA
  • LSA Type 5: OSPF ASBR External LSA
  • LSA Type 6: OSPF Group Membership LSA
  • LSA Type 7: OSPF Not So Stubby Area (NSSA) External LSA
  • LSA Type 8: OSPF External Attributes LSA (OSPFv2) / Link Local LSA (OSPFv3)
  • LSA Type 9: OSPF Link Scope Opaque (OSPFv2) / Intra Area Prefix LSA (OSPFv3)
  • LSA Type 10: OSPF Area Scope Opaque LSA
  • LSA Type 11:OSPF AS (Autonomous System) Scope Opaque LSA

The LSA payload varies in size according to the LSA type and the information it includes. The diagram below clearly shows how LSAs are contained within LSUs:

OSPF LSA Types

Figure 1. LSA Types contained within an OSPF LSU packet

As mentioned, OSPF currently supports 11 types of LSAs. Each LSA is used within specific boundaries of an OSPF network.

OSPF concepts, including router roles such as Designated Router (DR), Area Border Router (ABR), Autonomous System Border Router (ASBR), OSPF Areas and more, are analyzed in great depth in our article OSPF Basic Concepts – OSPF Areas – Router Roles. This article assumes the reader has a good understanding of basic OSPF theory and is comfortable with OSPF concepts.

LSA Type 1 – OSPF Router LSA

LSA Type 1 (Router LSA) packets are sent between routers within the same area of origin and do not leave the area. An OSPF router uses LSA Type 1 packets to describe its own interfaces but also carries information about its neighbors to adjacent routers in the same area.

LSA Type 1 Packets exchanged between OSPF routers within the same area

Figure 2. LSA Type 1 Packets exchanged between OSPF routers within the same area

LSA Type 2 – OSPF Network LSA

LSA Type 2 (Network LSA) packets are generated by the Designated Router (DR) to describe all routers connected to its segment directly. LSA Type 2 packets are flooded between neighbors in the same area of origin and remain within that area.

 LSA Type 2 Packets exchanged between OSPF DR and neighbor routers

Figure 3. LSA Type 2 Packets exchanged between OSPF DR and neighbor routers

LSA Type 3 – OSPF Summary LSA

LSA Type 3 (Summary LSA) packets are generated by Area Border Routers (ABR) to summarize its directly connected area, and advertise inter-area router information to other areas the ABR is connected to, with the use of a summary prefix (e.g 192.168.0.0/22). LSA Type 3 packets are flooded to multiple areas throughout the network and help with OSPF’s scalability with the use of summary prefixes.

 LSA Type 3 - An OSPF ABR router advertises the summarized route 192.168.2.0/24 to Area 0

Figure 4. LSA Type 3 - An OSPF ABR router advertises the summarized route 192.168.2.0/24 to Area 0

Looking at the diagram above, ABR router R2 creates a Type 3 Summary LSA and floods it into Area 0. In a similar way, ABR router R3 creates a Type 3 Summary LSA and floods it into Area 2. Type 3 Summary LSAs appear as O IA entries in the router routing table.

LSA Type 4 – OSPF ASBR Summary LSA

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